Recent rapid developments in electronic technology have led to the down-scaling of semiconductor devices. Since semiconductor devices require fast operation speeds and operation accuracy as well, various studies for optimizing the structures of down-scaled transistors included in the semiconductor devices are being carried out. In particular, as the size of a transistor is reduced the gate length may also be reduced but in order to maintain desired performance and suppress the occurrence of leakage currents, it is important to maintain a constant gate length.